Liquid crystal lens with enhanced electrical drive

ABSTRACT

Adaptive spectacles include a spectacle frame and first and second electrically-tunable lenses, mounted in the spectacle frame and having respective focal powers and optical centers that are determined by control voltages applied thereto. Control circuitry is configured to apply the control voltages so as to shift the optical centers of the electrically-tunable lenses responsively to the focal powers of the electrically-tunable lenses.

CROSS-REFERENCE TO RELATED APPLICATION

This application is a division of U.S. patent application Ser. No.17/521,887, filed Nov. 9, 2011, which is a division of U.S. patentapplication Ser. No. 16/081,927 (now U.S. Pat. No. 11,221,500), filedSep. 3, 2018, in the national phase of PCT Patent ApplicationPCT/IB2017/051943, filed Apr. 5, 2017, which claims the benefit of U.S.Provisional Patent Application 62/323,708, filed Apr. 17, 2016; U.S.Provisional Patent Application 62/330,265, filed May 2, 2016; U.S.Provisional Patent Application 62/350,723, filed Jun. 16, 2016; and U.S.Provisional Patent Application 62/394,770, filed Sep. 15, 2016. Thedisclosures of all these related applications are incorporated herein byreference.

FIELD OF THE INVENTION

The present invention relates generally to optical devices, andparticularly to electrically-tunable lenses.

BACKGROUND

Tunable lenses are optical elements whose optical characteristics, suchas the focal length and/or the location of the optical axis, can beadjusted during use, typically under electronic control. Such lenses maybe used in a wide variety of applications. For example, U.S. Pat. No.7,475,985 describes the use of an electro-active lens for the purpose ofvision correction.

Electrically-tunable lenses typically contain a thin layer of a suitableelectro-optical material, i.e., a material whose local effective indexof refraction changes as a function of the voltage applied across thematerial. An electrode or array of electrodes is used to apply thedesired voltages in order to locally adjust the refractive index to thedesired value. Liquid crystals are the electro-optical material that ismost commonly used for this purpose (wherein the applied voltage rotatesthe molecules, which changes the axis of birefringence and thus changesthe effective refractive index); but other materials, such as polymergels, with similar electro-optical properties can alternatively be usedfor this purpose.

Some tunable lens designs use an electrode array to define a grid ofpixels in the liquid crystal, similar to the sort of pixel grid used inliquid-crystal displays. The refractive indices of the individual pixelsmay be electrically controlled to give a desired phase modulationprofile. (The term “phase modulation profile” is used in the presentdescription and in the claims to mean the distribution of the localphase shifts that are applied to light passing through the layer as theresult of the locally-variable effective refractive index over the areaof the electro-optical layer of the tunable lens.) Lenses using gridarrays of this sort are described, for example, in the above-mentionedU.S. Pat. No. 7,475,985.

PCT International Publication WO 2014/049577, whose disclosure isincorporated herein by reference, describes an optical device comprisingan electro-optical layer, having an effective local index of refractionat any given location within an active area of the electro-optical layerthat is determined by a voltage waveform applied across theelectro-optical layer at the location. An array of excitationelectrodes, including parallel conductive stripes extending over theactive area, is disposed over one or both sides of the electro-opticallayer. Control circuitry applies respective control voltage waveforms tothe excitation electrodes and is configured to concurrently modify therespective control voltage waveforms applied to excitation electrodes soas to generate a specified phase modulation profile in theelectro-optical layer.

U.S. Patent Application Publication 2012/0133891 describes anelectro-optical apparatus and method for correcting myopia that includesat least one adaptive lens, a power source, and an eye tracker. The eyetracker includes an image sensor and a processor operatively connectedto the adaptive lens and the image sensor. The processor is configuredto receive electrical signals from the image sensor and to control thecorrection power of the adaptive lens to correct myopia, with thecorrection power dependent on a user's gaze distance and myopiaprescription strength.

As another example, U.S. Patent Application Publication 2012/0120333described a liquid crystal lens, a controlling method thereof and a3-Dimensional (3D) display using the same. The liquid crystal lensincludes a pair of electrode structures, which are arranged apart fromeach other, and a liquid crystal layer, which is arranged between thepair of electrode structures and includes a plurality of liquid crystalmolecules aligned in an initial aligning direction in which the liquidcrystal layer has a non-lens effect. The pair of electrode structuresare arranged to generate a first electric field, which is used to changealigning directions of the liquid crystal molecules to make the liquidcrystal layer have a lens effect. The pair of electrode structures arefurther arranged to generate a second electric field, which is used tomake the liquid crystal molecules revert to the initial aligningdirection.

PCT International Publication WO 2015/186010, whose disclosure isincorporated herein by reference, describes adaptive spectacles, whichinclude a spectacle frame and first and second electrically-tunablelenses, mounted in the spectacle frame. In one embodiment, controlcircuitry is configured to receive an input indicative of a distancefrom an eye of a person wearing the spectacles to an object viewed bythe person, and to tune the first and second lenses in response to theinput.

SUMMARY

Embodiments of the present invention that are described hereinbelowprovide improved electrically-tunable optical devices.

There is therefore provided, in accordance with an embodiment of theinvention, an optical device, including an electro-optical layer, havingan effective local index of refraction at any given location within anactive area of the electro-optical layer that is determined by a voltagewaveform applied across the electro-optical layer at the location.Conductive electrodes are disposed over opposing first and second sideof the electro-optical layer. Control circuitry is configured to applycontrol voltage waveforms between the conductive electrodes so as togenerate a phase modulation profile in the electro-optical layer thatcauses rays of optical radiation that are incident on the device toconverge or diverge with a given focal power, while varying an amplitudeof the control voltage waveforms for the given focal power responsivelyto an angle of incidence of the rays that impinge on the device from adirection of interest.

In some embodiments, the electro-optical layer includes a liquidcrystal.

In a disclosed embodiment, the control circuitry is configured to applya control voltage waveform at a predetermined amplitude in order toproduce a given phase shift in the rays that are incident along a normalto the device, and to apply the control voltage waveform at a firstamplitude, which is less than the predetermined amplitude, to producethe given phase shift in the rays that are incident at an acute angle tothe device at a first azimuth, and to apply the control voltage waveformat a second amplitude, which is greater than the predeterminedamplitude, to produce the given phase shift in the rays that areincident at the acute angle to the device at a second azimuth, which isopposite to the first azimuth.

Additionally or alternatively, the control circuitry is configured tovary the respective amplitudes of the control voltage waveforms over anarea of the device responsively to a mapping of the angle of incidenceof the rays over the area of the device. In some embodiments, the deviceis included in a spectacle lens, wherein the mapping is indicative ofthe angle at which the rays pass through the electro-optical layer tothe eye at each point over the area of the device.

In a disclosed embodiment, the control circuitry is configured to changethe device from a first focal power to a second focal power byconcurrently applying overshoot control voltages to each of a pluralityof the conductive electrodes for different, respective transitionperiods, followed by application of the control voltage waveformscorresponding to the second focal power.

There is also provided, in accordance with an embodiment of theinvention, an optical device, including an electro-optical layer, havingan effective local index of refraction at any given location within anactive area of the electro-optical layer that is determined by a voltagewaveform applied across the electro-optical layer at the location.Conductive electrodes are disposed over opposing first and second sideof the electro-optical layer. Control circuitry is configured to applyat least first control voltage waveforms and second control voltagewaveforms between the conductive electrodes so as to generate respectivefirst and second phase modulation profiles in the electro-optical layer,which cause rays of optical radiation that are incident on the device toconverge or diverge with respective first and second focal powers, andis configured to change from the first focal power to the second focalpower by concurrently applying overshoot control voltages to each of aplurality of the conductive electrodes for different, respectivetransition periods, followed by application of the second controlvoltage waveforms.

In some embodiments, the transition periods include a plurality of timeslots, and the overshoot control voltages applied to at least some ofthe plurality of the conductive electrodes include at least a firstovershoot voltage applied during a first time slot and a secondovershoot voltage applied during a second time slot.

Additionally or alternatively, the overshoot control voltages applied toat least some of the plurality of the conductive electrodes include apredefined high voltage, which is applied to different ones of theconductive electrodes for different, respective periods within thetransition periods.

Further additionally or alternatively, the control circuitry is furtherconfigured, upon changing from the first focal power to the second focalpower, to concurrently apply undershoot control voltages to at leastsome of the conductive electrodes before application of the secondcontrol voltage waveforms. In one embodiment, when the second focalpower is zero, the overshoot control voltages include a predefined highvoltage, which is applied by the control circuitry to all of theconductive electrodes on the first side of the electro-optical layer,followed by application of a predefined low voltage in the secondcontrol voltage waveforms.

In a disclosed embodiment, the overshoot control voltages applied to atleast one electrode among the plurality of the conductive electrodesdepend both on the first and second control voltage waveforms that areapplied to the at least one electrode and on the control voltagewaveforms that are applied to one or more other conductive electrodesthat are adjacent to the at least one electrode.

There is additionally provided, in accordance with an embodiment of theinvention, an optical device, including an electro-optical layer, havingan effective local index of refraction at any given location within anactive area of the electro-optical layer that is determined by a voltagewaveform applied across the electro-optical layer at the location.Conductive electrodes are disposed over opposing first and second sideof the electro-optical layer. Control circuitry is configured to applycontrol voltage waveforms between the conductive electrodes so as togenerate a phase modulation profile in the electro-optical layer thatcauses rays of optical radiation that are incident on the device toconverge or diverge with a given focal power, and is configured tochange from the given focal power to zero focal power by concurrentlyapplying a predefined high voltage to all of the conductive electrodeson the first side of the electro-optical layer, followed by applicationof a predefined low voltage thereto.

There is further provided, in accordance with an embodiment of theinvention, an optical device, including an electro-optical layer, havingan effective local index of refraction at any given location within anactive area of the electro-optical layer that is determined by a voltagewaveform applied across the electro-optical layer at the location.Conductive electrodes are disposed over opposing first and second sideof the electro-optical layer. Control circuitry is configured to applyat least first control voltage waveforms and second control voltagewaveforms between the conductive electrodes so as to generate respectivefirst and second phase modulation profiles in the electro-optical layer,which cause rays of optical radiation that are incident on the device toconverge or diverge with respective first and second focal powers, andis configured to change from the first focal power to the second focalpower by concurrently applying overshoot control voltages to each of aplurality of the conductive electrodes, followed by application of thesecond control voltage waveforms. The overshoot control voltages appliedto at least one electrode among the plurality of the conductiveelectrodes depend both on the first and second control voltage waveformsthat are applied to the at least one electrode and on the controlvoltage waveforms that are applied to one or more other conductiveelectrodes that are adjacent to the at least one electrode.

In a disclosed embodiment, the first and second phase modulationprofiles include multiple Fresnel phase transitions, which are arrangedso that the device operates as a Fresnel lens, and the overshoot controlvoltages are applied to the conductive electrodes in a vicinity of theFresnel phase transitions with a dependence on the voltage waveformsthat are applied to the adjacent conductive electrodes that differs fromthe overshoot control voltages applied to the conductive electrodes thatare not in the vicinity of the Fresnel phase transitions.

There are moreover provided, in accordance with an embodiment of theinvention, adaptive spectacles, including a spectacle frame and firstand second electrically-tunable lenses, mounted in the spectacle frameand having respective focal powers and optical centers that aredetermined by voltage waveforms applied thereto. Control circuitry isconfigured to receive an input indicative of a distance from an eye of aperson wearing the spectacles to an object viewed by the person, andresponsively to the distance to modify the voltage waveforms so as bothto tune the focal powers and to shift the optical centers of theelectrically-tunable lenses.

In a disclosed embodiment, the control circuitry is configured to shiftthe optical centers of the first and second electrically-tunable lensesdownward when the distance is less than a predefined threshold distance.Additionally or alternatively, the control circuitry is configured toreduce a distance between the optical centers of the first and secondelectrically-tunable lenses when the distance is less than a predefinedthreshold distance.

There is furthermore provided, in accordance with an embodiment of theinvention, an optical method, which includes providing an opticaldevice, which includes an electro-optical layer, having an effectivelocal index of refraction at any given location within an active area ofthe electro-optical layer that is determined by a voltage waveformapplied across the electro-optical layer at the location, and conductiveelectrodes disposed over opposing first and second side of theelectro-optical layer. Control voltage waveforms are applied between theconductive electrodes so as to generate a phase modulation profile inthe electro-optical layer that causes rays of optical radiation that areincident on the device to converge or diverge with a given focal power,while varying an amplitude of the control voltage waveforms for thegiven focal power responsively to an angle of incidence of the rays thatimpinge on the device from a direction of interest.

There is also provided, in accordance with an embodiment of theinvention, an optical method, which includes providing an opticaldevice, which includes an electro-optical layer, having an effectivelocal index of refraction at any given location within an active area ofthe electro-optical layer that is determined by a voltage waveformapplied across the electro-optical layer at the location, and conductiveelectrodes disposed over first and second side of the electro-opticallayer. First control voltage waveforms are applied between theconductive electrodes so as to generate a first phase modulation profilein the electro-optical layer, which causes rays of optical radiationthat are incident on the device to converge or diverge with a firstfocal power. In preparation for changing from the first focal power to asecond focal power, overshoot control voltages are concurrently appliedto each of a plurality of the conductive electrodes for different,respective transition periods. Subsequent to the overshoot controlvoltages, second control voltage waveforms are applied between theconductive electrodes so as to generate a second phase modulationprofile in the electro-optical layer, which causes the rays of theoptical radiation that are incident on the device to converge or divergewith the second focal power.

There is additionally provided, in accordance with an embodiment of theinvention, an optical method, which includes providing an opticaldevice, which includes an electro-optical layer, having an effectivelocal index of refraction at any given location within an active area ofthe electro-optical layer that is determined by a voltage waveformapplied across the electro-optical layer at the location, and conductiveelectrodes disposed over first and second side of the electro-opticallayer. Control voltage waveforms are applied between the conductiveelectrodes so as to generate a phase modulation profile in theelectro-optical layer that causes rays of optical radiation that areincident on the device to converge or diverge with a given focal power.The device is changed from the given focal power to zero focal power byconcurrently applying a predefined high voltage to all of the conductiveelectrodes on the first side of the electro-optical layer, followed byapplication of a predefined low voltage thereto.

There is further provided, in accordance with an embodiment of theinvention, an optical method, which includes providing an opticaldevice, which includes an electro-optical layer, having an effectivelocal index of refraction at any given location within an active area ofthe electro-optical layer that is determined by a voltage waveformapplied across the electro-optical layer at the location, and conductiveelectrodes disposed over first and second side of the electro-opticallayer. First control voltage waveforms are applied between theconductive electrodes so as to generate a first phase modulation profilein the electro-optical layer, which causes rays of optical radiationthat are incident on the device to converge or diverge with a firstfocal power. In preparation for changing from the first focal power to asecond focal power, overshoot control voltages are concurrently appliedto each of a plurality of the conductive electrodes. Subsequent to theovershoot control voltages, second control voltage waveforms are appliedbetween the conductive electrodes so as to generate a second phasemodulation profile in the electro-optical layer, which causes the raysof the optical radiation that are incident on the device to converge ordiverge with the second focal power. The overshoot control voltagesapplied to at least one electrode among the plurality of the conductiveelectrodes depend both on the first and second control voltage waveformsthat are applied to the at least one electrode and on the controlvoltage waveforms that are applied to one or more other conductiveelectrodes that are adjacent to the at least one electrode.

There is moreover provided, in accordance with an embodiment of theinvention, an optical method, which includes providing spectaclesincluding first and second electrically-tunable lenses mounted in aspectacle frame and having respective focal powers and optical centersthat are determined by voltage waveforms applied thereto. An inputindicative of a distance from an eye of a person wearing the spectaclesto an object viewed by the person is received. Responsively to thedistance, the voltage waveforms are automatically modified so as both totune the focal powers and to shift the optical centers of theelectrically-tunable lenses.

There are furthermore provided, in accordance with an embodiment of theinvention, adaptive spectacles, including a spectacle frame and firstand second electrically-tunable lenses, mounted in the spectacle frameand having respective focal powers and optical centers that aredetermined by voltage waveforms applied thereto. Control circuitry isconfigured to apply control voltage waveforms so as to shift the opticalcenters of the electrically-tunable lenses responsively to the focalpowers of the electrically-tunable lenses.

In some embodiments, the control circuitry is configured to shift theoptical centers of the first and second electrically-tunable lensesdownward and/or to reduce a distance between the optical centers of thefirst and second electrically-tunable lenses when the focal powers ofthe electrically-tunable lenses are increased.

The present invention will be more fully understood from the followingdetailed description of the embodiments thereof, taken together with thedrawings in which:

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic, pictorial illustration of adaptive spectacles, inaccordance with an embodiment of the invention;

FIG. 2 is a schematic sectional view of an electrically-tunable opticalphase modulator, in accordance with an embodiment of the invention;

FIG. 3 is a schematic sectional view of rays incident at differentangles on an electrically-tunable lens, in accordance with an embodimentof the invention;

FIG. 4 is a plot that schematically shows a relation between phasemodulation by an electrically-tunable lens and applied voltage fordifferent angles of incidence, in accordance with an embodiment of theinvention;

FIG. 5 is a schematic side view of an electrically-tunable lensillustrating a relation between angle of incidence and beam axis, inaccordance with an embodiment of the invention;

FIG. 6A is a plot that schematically illustrates a phase modulationprofile of an electrically-tunable lens, in accordance with anembodiment of the invention;

FIG. 6B is a plot that schematically shows a relation between the phasemodulation profile of FIG. 6A and the voltage applied to theelectrically-tunable lens in order to generate the corresponding phasemodulation for different angles of incidence, in accordance with anembodiment of the invention;

FIG. 7A is a plot that schematically illustrates voltage waveformsapplied over time to an electrically-tunable lens in order to modify aphase modulation in the lens, in accordance with an embodiment of theinvention;

FIG. 7B is a plot that schematically illustrates the phase modulationachieved over time by applying the waveforms of FIG. 7A;

FIG. 8A is a plot that schematically illustrates voltage waveformsapplied over time to an electrically-tunable lens in order to modify aphase modulation in the lens, in accordance with another embodiment ofthe invention;

FIG. 8B is a plot that schematically illustrates the phase modulationachieved over time by applying the waveforms of FIG. 8A; and

FIG. 9 is a plot that schematically illustrates phase modulationsachieved over time by application of different voltage waveforms to anelectrically-tunable lens, in accordance with an embodiment of theinvention.

DETAILED DESCRIPTION OF EMBODIMENTS System Description

FIG. 1 is a schematic, pictorial illustration of adaptive spectacles 20,in accordance with an embodiment of the invention. Spectacles 20comprise electrically-tunable lenses 22 and 24, mounted in a frame 25.The optical properties of the lenses, including focal length and opticalcenter (or equivalently, the location of the optical axis) arecontrolled by control circuitry 26, which is powered by a battery 28 orother power source. Control circuitry 26 typically comprises an embeddedmicroprocessor with hard-wired and/or programmable logic components andsuitable interfaces for carrying out the functions that are describedherein. These and other elements of spectacles 20 are typically mountedon or in frame 25, or may alternatively be contained in a separate unit(not shown) connected by wire to frame 25.

In some embodiments, lenses 22 and 24 are compound lenses, whichcomprise multiple elements: For example, each of lenses 22 and 24 maycomprise a fixed lens, typically made from glass or plastic, providing abaseline optical power, which is modified dynamically by one or moreelectrically-tunable optical phase modulators that are integrated withthe fixed lens. (For this reason, lenses 22 and 24 can themselves beconsidered electrically-tunable lenses.) Alternatively, lenses 22 and 24may each comprise only a single electrically-tunable element, and thefixed lens may not be needed in some applications. Lenses 22 and 24 mayeach comprise a pair of electrically-tunable cylindrical lenses, withorthogonal cylinder axes. Alternatively, lenses 22 and 24 may eachcomprise a single electrically-tunable element, which is configured togenerate two-dimensional phase modulation profiles and thus emulatespherical or aspheric lenses (or their Fresnel equivalents). Both ofthese sorts of lens configurations, as well as waveforms for driving thelenses, are described in detail in the above-mentioned WO 2014/049577and WO 2015/186010.

In some embodiments, lenses 22 and 24 comprise two (or more)electrically-tunable elements with polarization-dependentelectro-optical layers, which are oriented so as to refractmutually-orthogonal polarizations. Alternatively, lenses 22 and 24 maycomprise polarization-independent electro-optical layers, for example asdescribed in PCT Patent Application PCT/IB2017/051435, filed Mar. 13,2017, whose disclosure is incorporated herein by reference.

In some embodiments, spectacles 20 comprise one or more sensors, whichsense the distance from an eye 31 of the person wearing the spectaclesto an object 34 viewed by the person. Control circuitry 26 tunes lenses22 and 24 according to the sensor readings. In the pictured example, thesensors include a pair of eye trackers 30, which detect respective gazedirections 32 of right and left eyes 31. Control circuitry 26 typicallyshifts the respective optical axes of lenses responsively to the sensedgaze directions. Furthermore, the control circuitry can use the distancebetween the pupils, as measured by eye trackers 30, to estimate theuser's focal distance (even without analyzing the actual gazedirection), and possibly to identify the distance between the eye andobject 34.

Additionally or alternatively, a camera 36 captures an image of object34, for use by control circuitry 26 in identifying the object andsetting the focal distance. Either eye trackers 30 or camera 36 may beused in determining the focal distance, but both of these sensors can beused together to give a more reliable identification of the object.Alternatively or additionally, camera 36 may be replaced or supplementedby a rangefinder or other proximity sensor, which measures the distanceto object 34.

In some embodiments, spectacles 20 also include at least one triggersensor 38, which activates the other components of spectacles 20. Forexample, trigger sensor 38 may comprise a timer that triggers controlcircuitry 26 and other elements periodically, or other sensorsindicating a possible change of the viewing distance, such as a headmovement sensor, or a user input sensor.

Lenses 22 and 24 typically have better optical quality in the opticalcenter of the lens than in the lens periphery. (The optical center isthe point on the lens through which the optical axis passes, i.e., theaxis of symmetry of the phase modulation profile of the lens, which canbe shifted in an electrically-tunable lens by changing the voltagewaveforms that drive the lens.) Therefore, it is beneficial to centerthe lenses opposite the pupils and to move the optical center of thelens dynamically, by modifying the voltage waveforms applied to theelectrically-tunable lenses so that the optical center of the lens isalways opposite the pupil.

Alternatively, when lenses 22 and 24 are used as dynamic focalspectacles for presbyopia, the optical center can be predefined as afunction of optical power or equivalently, of distance to the object, asindicated by an input from eye trackers 30 and/or camera 36, forexample. In general, the distance between the lens centers for far viewis determined by the pupil distance of the wearer. When the wearer islooking at a closer distance and requires focus adjustment, controlcircuitry 26 can modify the voltage waveforms so as to switch lenses 22and 24 to the correct optical power while reducing the distance betweenthe lens center positions, to adjust for the fact that when a personlooks at a closer distance the distance between the pupils is decreased.Additionally or alternatively, control circuitry 26 can adjust theheight of the optical centers of the lenses, for example by shifting theoptical centers of lenses 22 and 24 downward when the distance to theobject is less than a predefined threshold distance, to reflect thetendency of people to look through a lower part of the spectacles whenviewing close objects.

FIG. 2 is a schematic sectional view of an optical phase modulator 40,which defines the active area of an electrically-tunable lens (such aslens 22 or 24), in accordance with an embodiment of the invention. Phasemodulator 40 comprises an electro-optical layer 46, sandwiched betweenan upper substrate 42 and a lower substrate 44, which comprise atransparent material, for example, glass. Layer 46 comprises a liquidcrystal material, which is typically contained by suitableencapsulation, as is known in the art. Substrates 42 and 44 can becoated on their insides with a polyimide alignment layer 54 (for examplePI-2555, produced by Nissan Chemical Industries Ltd., Japan), whichcauses liquid crystal molecules 48 to line up in a desired parallelorientation.

Conductive electrodes 50 and 52 are disposed over opposing first andsecond sides of electro-optical layer 46. Electrodes 50 and 52 comprisea transparent, conductive material, such as indium tin oxide (ITO), asis known in the art, which is deposited on the surfaces of substrates 42and 44, respectively. (Alternatively, non-transparent excitationelectrodes may be used, as long as they are thin enough so that they donot cause disturbing optical effects.) Although for the sake of visualclarity, only a few electrodes are shown in FIG. 2 , in practice, forgood optical quality, optical phase modulator 40 will typically compriseat least 100 stripe electrodes for excitation, and possibly even 400 ormore.

Electrodes 50 in the pictured embodiment are arranged as an array ofparallel stripes. On the opposite side of layer 46, electrodes 52 maycomprise stripes perpendicular to electrodes 50, which enable controlcircuitry 26 to apply two-dimensional voltage patterns across layer 46.Alternatively, electrode 52 may comprise a uniform layer on substrate44, functioning as an electrical ground plane. In this latter case, onlyone-dimensional voltage patterns can be applied across layer 46, whichcan be used to create phase modulation profiles equivalent tocylindrical lenses. Two such optical phase modulators 40 in series, withelectrodes 50 oriented orthogonally one to the other, can be used ineach of lenses 22 and 24 to generate two-dimensional optical modulationpatterns.

Due to the behavior of liquid crystal molecules 48, electro-opticallayer 46 has an effective local index of refraction at any givenlocation within the active area of the layer that is determined by thevoltage waveform that is applied across the electro-optical layer atthat location. Control circuitry 26 is coupled to electrodes 50 and 52and applies the appropriate control voltage waveforms to the electrodesso as to modify the optical phase modulation profile of theelectro-optical layer 46. When used in spectacles, the phase modulationprofile is chosen to emulate a lens, causing rays of optical radiationthat are incident on optical phase modulator 40 to converge or divergewith a desired focal power. For strong focal power, the phase modulationprofile may comprise a Fresnel profile, with sharp peaks and troughs.Alternatively or additionally, the control voltage waveforms may bechosen so as to give rise to a smooth refractive phase modulationprofile.

Further details of a variety of electrode structures that can be used inelectrically-tunable lenses, as well as the control voltage waveformsthat may be applied to such electrodes in order to generate varioussorts of phase modulation profiles, are described in the above-mentionedWO 2014/049577 and WO 2015/186010. These details are omitted here forthe sake of brevity.

Adjustment of Control Voltage Waveforms for Angle of Incidence

When a lens is positioned in front of a human eye, the area of the lensthrough which light passes and reaches the central vision area on theretina (the fovea) depends on the angle at which the person is looking.When the person looks straight ahead, light reaching the fovea typicallypasses the lens through the central area of the lens, in a directiongenerally normal to the lens. When the person looks to the side, the eyerotates, the pupil moves, and the light reaching the fovea passesthrough a different region in the lens, typically at an acute angle tothe lens surface. Thus, different areas of the lens are used primarilyto refract light that is incident at different angles. The embodimentsthat are described in this section of the present patent applicationoptimize the performance of an electrically-tunable lens to account forthis point.

FIG. 3 is a schematic sectional view of rays incident at differentangles on optical phase modulator 40 in an electrically-tunable lens,such as lens 22 or 24, in accordance with an embodiment of theinvention. In this figure, a certain control voltage has been appliedbetween electrodes 50 and 52, thus changing the angle of orientation ofliquid crystal molecules 48, and hence changing the effective index ofrefraction of electro-optical layer 46.

FIG. 3 illustrates conceptually the difference in the effectiverefractive index for light that is incident along a normal to modulator40, marked L1, in comparison with light incident at an acute angle fromtwo different, opposing azimuths, marked L2 and L3. For light incidentat the angle L2, the effective refractive index is different from thatof L1, due to the different angle between the propagation axis (whichdetermines the direction of the electrical field of the incident light)and the director axis of liquid crystal molecules 48. For light incidentat the angle L3, the refractive index is different from that of both L1and L2. Therefore, for each incidence angle α there is a different phasevs. voltage graph, defined by the function ϕ=T_(α)(V).

FIG. 4 is a plot that schematically shows the relation between phasemodulation by modulator 40 as a function of applied voltage V betweenelectrodes 50 and 52 for different angles of incidence, in accordancewith an embodiment of the invention. The phase modulation was measuredusing an interferometer for three different angles of incidence α=0, 10,and −10 degrees, giving curves 60, 62 and 64, respectively. (The values10 and −10 refer to incidence at 10° from the normal at opposingazimuths.) The relation between voltage and phase modulation, T_(α)(V),can be measured in this manner over a range of incidence angles ofinterest, and intermediate angular values can readily be interpolatedfrom the measured values.

FIG. 5 is a schematic side view of electrically-tunable lens 24 and eye31, illustrating the relation between the areas of lens 24 through whichbeams pass and the angle of incidence of the beams on the lens, inaccordance with an embodiment of the invention. The figure illustrateshow different areas of the lens are used by eye 31 to view objects indifferent directions. In a liquid crystal lens, it is thus desirable tooptimize the modulation of the liquid crystal in each area to accountfor the incidence angle of rays that impinge on the lens from thedirection of interest. The “direction of interest” is defined generallyby the relation between points in the object region and thecorresponding points in the image plane of the lens. Thus, for example,in a spectacle lens, the direction of interest for any given area on thelens surface can be defined as the angle of a ray that passes throughthe area between a location on the eye (such as the pupil or an imagepoint on the retina) and a corresponding location in the object regiontoward which the person wearing the spectacles is looking. This relationbetween areas of lens 24 and corresponding directions of interest isexemplified by the rays shown in FIG. 5 .

For a given target phase modulation profile of a lens, ϕ(x), acorresponding voltage profile V(x) is applied, such thatϕ(x)=T_(α)(V(x)). Since T_(α) is not symmetrical, i.e., T_(α)≠T_(−α), asexplained above, a non-symmetrical voltage profile V(x) is needed inorder to achieve a symmetrical phase modulation profile ϕ(x), such as aspherical or aspherical lens, that extends over the entire field ofview. In other words, to achieve a particular symmetrical refractiveprofile, the voltage applied across electro-optical layer 46 at a givendistance from the lens center on one side of the field will be markedlygreater than at the same distance from the center on the other side ofthe field.

In one embodiment of the invention, lens 24 can be optimized for acertain viewing angle, for example straight ahead (i.e., Angle 2 in FIG.5 ). Lens 24 is positioned at a distance D from the pupil of eye 31. Thetarget phase modulation profile, for instance a profile that emulates anaspheric lens, is ϕ(x). Positions on the lens (X1, X2, X3, . . . ) aremapped to light incidence angles as shown in the figure, for exampleusing the formula α(x)=tan⁻¹(x/D). The voltage profile applied betweenelectrodes 50 and 52 is then translated using the phase vs. voltagemapping into a function of x and T_(α)(V): V(x)=T_(α) ⁻¹(ϕ(x)).

In an alternative embodiment, every position on lens 24 is optimized forthe incidence angle of light passing through that position in the lenswhen eye 31 is looking in that direction. The difference between thisembodiment and the previous embodiment is that in this case theoptimization also depends on the movement of the pupil as the person islooking to the side, leading to a different mapping between position onthe lens and light incident angle, α(x). For example, this mapping canbe estimated in the present case by α(x)=tan⁻¹(x/L), wherein L in thedistance between the center of rotation of the eye and the lens. Thecalculation of the required voltage profile is performed in the samemanner as in the previous embodiment, but with a different incidenceangle mapping α(x).

FIGS. 6A and 6B are plots that schematically illustrate the principlesof this approach in implementing a particular refractive phase profileacross lens 24, in accordance with an embodiment of the invention. FIG.6A illustrates a phase modulation profile 70, while FIG. 6B shows therelation between this phase modulation profile and the voltage appliedto optical phase modulator 40 in order to generate the correspondingphase modulation for different angles of incidence. A curve 72 shows therequired voltage as a function of position across modulator 40 for lightthat is normally incident (0 degrees) on phase modulator 40, whilecurves 74 and 76 show the respective modifications needed in the voltageprofile when the light is incident at −10 or +10 degrees.

The amplitudes of the control voltage waveforms (meaning the voltagelevels in the present embodiments) applied by control circuitry 26 for agiven focal power of lens 24 are thus adjusted to account for the anglesof incidence of the light rays on the lens. In the present example, theamplitudes vary over the area of lens 24 based on the mapping describedabove of the angles of incidence of the rays over the area of the lens.As explained earlier, the adjustment of control voltages for the angleis non-symmetrical, meaning that to achieve a given focal power, adifferent voltage will be applied at positive angles from that appliedat the corresponding negative angles to obtain identical phasemodulation.

As a specific example, assume that lens 24 is positioned at a distanceL=3 cm from the center of rotation of the eye. This example willillustrate how to calculate the applied voltage for three differentelectrodes, located at x=0 (lens center) and transverse displacementsx=−5.3 mm and x=+5.3 mm from the center. The required phase modulationpattern is taken to be a spherical lens with optical power of twodiopters, with a Fresnel structure: ϕ(x)=πx²/λf mod 2πm, wherein λ=0.5μm is the optical wavelength, f=0.5 m is the focal length, and n=9 isthe Fresnel structure height in radians.

For the center electrode:

The required phase modulation is ϕ(x=0)=0.

The incidence angle is α(x=0)=tan⁻¹(x/L)=0.

As shown in FIG. 4 , the required voltage is V=0 V (or V<0.5V).

For x=−5.3 mm:

The required phase is ϕ(x=−5.3 mm)=13.7 rad.

The incidence angle is: α(x=−5.3)=tan⁻¹(x/L)=−10°.

The required voltage from FIG. 4 is V=1.8 V.

For x=+5.3 mm:

The required phase is ϕ(x=+5.3 mm)=13.7 rad.

The incidence angle is: α(x=+5.3)=tan⁻¹(x/L)=+10°.

The required voltage from FIG. 4 is V=1.5 V.

As noted earlier, although the phase modulation profile is symmetrical,with ϕ(x)=ϕ(−x), the applied voltage profile is not symmetrical, i.e.,V(x)≠V(−x).

Although the example above is one-dimensional, it can be readilyextended to two dimensions by applying the sorts of modificationsdescribed above to the two-dimensional modulation configurations thatare described in the above-mentioned WO 2014/049577 and WO 2015/186010.

Reduction of Switching Latency Using Overdrive

As explained above, control circuitry 26 applies different controlvoltage waveforms between conductive electrodes 50 and 52 in order togenerate different phase modulation profiles, which cause rays ofoptical radiation that are incident on the lenses 22 and 24 to convergeor diverge with different, respective focal powers. The phase modulationis achieved by rotation of liquid crystal molecules 48, and changing thevoltage changes the rotation angle and hence the effective localrefractive index.

Because of the nature of the liquid crystal material, however, there canbe substantial latency in rotation of the molecules following a changein applied voltage. This latency, in turn, can lead to a noticeabledelay in accommodation of spectacles 20 to changes in viewing distanceand/or angle of eyes 31. Therefore, in the embodiments that aredescribed in this section, control circuitry 26 uses overdrivetechniques to reduce the transition time between different focal powers.In other words, in switching between first and second sets of controlvoltage waveforms, corresponding to two different focal powers, controlcircuitry 26 first applies overshoot or undershoot voltages to certainelectrodes 50 over certain transition periods, and only then applies thesecond set of control voltage waveforms. The degree and timing of theovershoot and/or undershoot (i.e., the amplitudes and periods ofapplication of the overshoot or undershoot voltages) can vary fromelectrode to electrode, depending upon the initial and final voltagewaveforms applied to each electrode; but it is a feature of the devicearchitecture provided by the present embodiments that the differentovershoot and undershoot voltages can be applied concurrently andindependently to the different electrodes.

In some of the present embodiments, control circuitry 26 generates therequired overshoot or undershoot by applying a predefined high or lowvoltage to at least some of electrodes 50 for different, respective timedurations. (The voltages are “high” or “low” relative to the range ofvoltages applied over all the electrodes in steady state and maycomprise, for example, the maximum and minimum voltage levels,respectively, that can be applied by control circuitry 26.) This schemeis relatively simple to implement, while minimizing the time requiredfor the change in focal power. For example, let us assume that drivingoptical phase modulator 40 to create a focal power D1 requires applyinga voltage V1(i) to each electrode i, while focal power D2 requiresapplying voltages V2(i). If for a given electrode k, V1(k)<V2(k), thenupon initiating the change from D1 to D2, control circuitry 26 willfirst change the voltage applied on electrode k to Vmax−a predefinedhigh voltage that is typically equal to the maximal voltage that can beapplied to the electrode. The voltage is held at Vmax for a period T1.Similarly, if V1(k)>V2(k), the voltage applied on electrode k is firstchanged to Vmin—a predefined low voltage typically equal to the minimalvoltage that can be applied to the electrode—for a period T2, whoseduration similarly depends on the initial voltage V1(k) and the finalvoltage V2(k).

As the durations of T1 and T2 depend on the initial voltage V1(k) andthe final voltage V2(k), control circuitry 36 can use a look-up table(LUT) to hold the required overdrive periods T(V1,V2). After theappropriate period T in each case, as indicated by the value in the LUT,the voltage of electrode k is set to V2(k). The period T(V1,V2) is setto be equal to the time it takes liquid crystal molecules 48 to rotatefrom their initial angle, due to the voltage V1, to a target anglecorresponding to voltage V2, while applying Vmax on the electrode.

The application of different transition periods to the differentelectrodes can be further simplified by the use of fixed time slots indefining the different transition periods. Thus, in some embodiments,the transition times T(V1,V2) are divided into multiple time slots, forexample five or more predefined time slots. A first overshoot voltage isapplied during one or more initial time slots, followed by a secondovershoot voltage applied during a subsequent time slot. For example,when switching from V1 to V2, the voltage can first be switched to Vmax(or Vmin if V2<V1) for a number of time slots, and then the voltage ischanged to a different, intermediate value Vx for a single time slot. Inthis case, two LUTs are used to determine the voltage waveform requiredin order to switch rapidly from V1 to V2: One LUT includes the number oftime slots over which the voltage is held at Vmax (or Vmin), and theother holds the intermediate value Vx(V1,V2).

As a specific example, assume that in switching from a certain V1 to acertain V2 with time slots of 1 ms, if the voltage is held at Vmax for 5ms, liquid crystal molecules 48 will not yet reach the required anglecorresponding to voltage V2. If the voltage is held at Vmax for 6 ms,however, the molecules may rotate more than is required for voltage V2.Therefore, for a transition from V1 to V2, the first LUT will containthe value 5 (5 ms of V=Vmax); and for the next time slot of 1 ms, thevoltage, read from the second LUT, will be Vx(V1,V2). Typically,Vx(V1,V2)>V2 (for the case V2>V1).

In another embodiment of the invention, the time slots are not uniformin length, but rather are graduated in order to increase the number ofelectrodes that reach their target values after shorter delays. Thisapproach can achieve better optical quality in less time. For instance,to reduce the required memory size and logic complexity, controlcircuitry 26 may be limited to ten time slots, with a total requiredoverdrive time (worst-case transitions) of 1 sec. Typically, however,most of the transitions require significantly less time than the worstcase. The duration of the time slots is therefore distributed unevenly,for example by dividing the ten available time slots into three slots of25 ms, followed by two slots of 50 ms, and then one slot each of 75 ms,100 ms, 150 ms, 200 ms and 300 ms.

The difference between this graduated scheme and a simple scheme usingten uniform slots of 100 ms each is that when the graduated scheme isused, fast transitions can be optimized to shorter times, thus achievingbetter lens quality faster. The fact that a small number of individualpixels in phase modulator 40 may take relatively longer to reach theirtarget phase angles is insignificant, since the lens quality depends onthe percentage of pixels that have reached the correct state. It istherefore useful to maximize the number of “correct” pixels within ashort time by using graduated time slots, even if other pixels will takelonger to settle.

FIGS. 7A and 7B are plots that schematically illustrate the applicationof overshoot voltage waveforms over time to optical phase modulator 40in order to modify the local phase modulation, in accordance with anembodiment of the invention. FIG. 7A shows a voltage waveform 80 withoutovershoot, along with a voltage waveform 82 in which an overshootvoltage is applied to one of electrodes 50 in two successive time slots.FIG. 7B shows a phase modulation curve 84 over time that results fromapplication of waveform 80, without overshoot, along with a phasemodulation curve 86 that is obtained by application of waveform 82. Theactual phase modulation characteristics were measured using a liquidcrystal panel with a total phase modulation of 61 radians and anoverdrive scheme using five time slots (two slots of 100 ms, followed bythree slots of 200 ms). The voltage waveforms applied to the liquidcrystal were AC voltages at a frequency of 1 kHz, and FIG. 7A thereforeshows the RMS voltage as a function of time.

In both of waveforms 80 and 82, the voltage was switched from 0.25 V to2.4 V, and the resulting phase modulation in curves 84 and 86 changedfrom 0 to 42 radians. Without overdrive, the change in phase modulationtook nearly 2 sec, as shown by curve 84. In waveform 82, the voltage wasfirst switched to the maximal voltage Vmax=4.7 V for one time slot of100 ms, and then the voltage was switched to an intermediate voltage of4.3 V for the second time slot of 100 ms. The voltage was switched tothe final value of 2.4 V after 200 ms. Thus, in the case of curve 86,the phase transition was much faster, about 200 ms. To implement thetransition illustrated by waveform 82 using the two LUTs describedabove, the first LUT will hold the value 1 (one time slot of maximalvoltage), and the second LUT will hold the value 4.3 (4.3 V, or someinteger index corresponding to 4.3 V).

FIGS. 8A and 8B are plots that schematically illustrate the applicationof undershoot voltage waveforms over time to optical phase modulator 40in order to modify the local phase modulation, in accordance withanother embodiment of the invention. FIG. 8A shows a voltage waveform 90without undershoot, along with a voltage waveform 92 in which anundershoot voltage is applied to one of electrodes 50 in two successivetime slots. FIG. 8B shows a phase modulation curve 94 over time thatresults from application of waveform 90, without undershoot, along witha phase modulation curve 96, which is obtained by application ofwaveform 92.

In this case, using waveform 90 without overdrive, the voltage wasswitched from 4.1 V to 1.8 V, and the phase modulation illustrated bycurve 94 changed from 39 to 3 radians in about 2 sec. Using theoverdrive scheme illustrated by waveform 92, the voltage was firstswitched to the minimal voltage Vmin=0.2 V for three time slots (2×100ms plus 1×200 ms, giving a total of 400 ms), after which the voltage wasswitched to an intermediate voltage of 1.7 V for the fourth time slot of200 ms, and then to the final value of 1.8 V after a total of 600 ms.The phase transition from 39 to 3 radians was much faster in this case,as illustrated by curve 96, taking only about 500 ms. To implementwaveform 92, the first LUT holds the value 3 (three time slots ofminimal voltage), and the second LUT holds the intermediate voltagevalue 1.7 (or some integer index corresponding to 1.7 V).

Typically, as can be seen in the preceding figures, switching times tohigher voltages are faster than switching times to lower voltages (i.e.,rise times are shorter than fall times). Therefore, it is beneficial,when possible, to use rise times rather than fall times, for examplewhen switching to zero focal power (turning the lens off). Although zeropower is normally achieved by applying the minimum voltage (for example,zero voltage) to all of electrodes 50, zero focal power can often bereached more rapidly by applying equal high voltages to all of theelectrodes.

Therefore, in another embodiment of the invention, control circuitry 26switches optical phase modulator 40 off (i.e., switches to zero focalpower), by concurrently applying a predefined maximum voltage, Vmax, toall of electrodes 50, followed by application of a predefined minimumvoltage to all the electrodes. Switching to the maximum voltage rapidlyturns off the focal power. Reducing the voltages to the minimumthereafter is not essential and can take place slowly. As long as thevoltage is reduced uniformly over all the electrodes, the focal power ofmodulator 40 will remain at zero. The reduction of the voltages isuseful in conserving power, as well as achieving faster transitions whenthe lens is turned back on.

A lens typically has a spatial phase modulation profile that isrelatively slow-varying near the center of the lens, and becomes steeperfarther from the lens center. For a dynamic lens based on optical phasemodulator 40 (or another, similar electrically-tunable device), however,the phase modulation profile is applied by discrete electrodes 50. Whenthe phase profile changes rapidly relative to the inter-electrode pitch(with a steep slope, as typically occurs far from the center ofmodulator 40), voltage differences between adjacent electrodes grow, andtherefore electrical crosstalk between the electrodes becomessignificant.

To mitigate this problem when the focal power of the lens is changed,control circuitry 26 can set the overdrive voltages (i.e., overshoot orundershoot) that are applied to each electrode in a manner that dependsnot only on the initial and final control voltage waveforms that areapplied to the electrode itself, but also on the control voltagewaveforms that are applied to the adjacent electrodes. Thus, forexample, when switching the voltage applied to electrode k from V1(k) toV2(k)>V1(k), control circuitry 26 first sets the voltage to Vod(k)>V2(k)for a period of T(k), wherein Vod(k) and/or T(k) depends on the initialand final voltages of electrode k, V1(k) and V2(k), as well as on theinitial and final voltages of the adjacent electrodes, V1(k−1), V2(k−1),V1(k+1), and V2(k+1). Similarly, if V1(k)<V2(k), then the undershootvoltage will be Vod(k)<V2(k), with a similar dependence on the adjacentelectrodes. When there is strong crosstalk, more than one neighbor oneach side can be considered when determining the overdrive duration.

Generally, both T(k) and Vod(k) can depend on the voltage transitions.The values of T(k) and Vod(k) can be stored in LUTs:

-   -   T(V1(k−1), V1(k), V1(k+1), V2(k−1), V2(k), V2(k+1)), and    -   Vod(V1(k−1), V1(k), V1(k+1), V2(k−1), V2(k), V2(k+1)).    -   Storing these six-dimensional LUTs, however, can consume a        substantial amount of memory.

The complexity and size of the LUTs can be reduced by assuming that thephase modulation profile of a lens changes smoothly (although thisassumption does not hold for a Fresnel lens near the Fresnel phasetransitions), and thus expressing T(k) and Vod(k) as functions of theslope of the phase modulation, rather than of the exact voltages of theadjacent electrodes. Therefore, in another embodiment of the invention,the overdrive voltage and/or overdrive duration when switching thevoltage of electrode k is determined by V1(k), V2(k), S1(k), and S2(k),wherein S1(k) and S2(k) are the initial and final voltage slopes aroundelectrode k, or alternatively, the initial and final slopes of the phasemodulation profile around electrode k. In this case the LUTs are onlyfour-dimensional: Vod(V1,V2,S1,S2) and T(V1,V2,S1,S2). The LUTs can beextended, as explained above, to include multiple overdrive time slots,with the overdrive voltage for each electrode in each time slotdepending on the initial and final voltages of that electrode as well ason the initial and final voltages of the neighboring electrodes.

When optical phase modulator 40 is driven to operate as a Fresnel lens,the resulting phase modulation profiles comprise multiple Fresnel phasetransitions, where the phase modulation is discontinuous. In this case,the overshoot control voltages that depend on the adjacent conductiveelectrodes can be applied preferentially to the conductive electrodesthat are in the vicinity of the Fresnel phase transitions. For theseelectrodes, for example, control circuitry 26 can use different LUTs,Vod_(F)(V1,V2,S1,S2) and T_(F)(V1,V2,S1,S2). The slopes here arecalculated over the vicinity of the phase transition, rather than in thetransition itself, which is discontinuous.

The optimal overdrive parameters, particularly for off-axis electrodesin locations of rapid phase transition, will vary depending on theproperties of electro-optical layer 46 and other features and dimensionsof optical phase modulator 40. It can be difficult to derive theoverdrive parameters a priori, but the LUTs can be populated in eachcase by a simple process of trial and error.

FIG. 9 is a plot that schematically illustrates phase modulation curves100, 102 and 104 over time that are achieved by application of differentvoltage waveforms to electrodes 50 of optical phase modulator 40, inaccordance with an embodiment of the invention. To obtain these curves,the average phase modulation was measured over time using a liquidcrystal panel with a 20 μm electrode pitch. This figure illustrates anempirical optimization of overdrive parameters to account for crosstalkbetween adjacent electrodes.

Curve 100 shows the phase modulation profile achieved using optimaloverdrive without crosstalk, as the same voltage is applied to allelectrodes. The voltage was switched from V1=1.84 V to V2=2.04 V.Without crosstalk, and assuming 100 ms overdrive duration, the optimalovershoot voltage was found to be 2.65 V, which resulted in the phasetransition illustrated by curve 100.

To generate curve 102, the panel was set to an initial voltage profilewith alternating voltages of 1.84 V and 2.04 V on even- and odd-numberedelectrodes, respectively, after which the voltage was switched to 2.04 Vacross all electrodes. The same overdrive scheme was used as in theprevious case (though only on the electrodes that were previously set to1.84 V), neglecting the crosstalk between adjacent electrodes. As shownby curve 102, the overdrive voltage was too high, and thus led to anundesirable overshoot in the phase modulation.

The overshoot voltage was then corrected to take the crosstalk intoaccount, resulting in the lower overshoot value of 2.59 V. As a result,shown in curve 104, the overshoot disappears.

It will be appreciated that the embodiments described above are cited byway of example, and that the present invention is not limited to whathas been particularly shown and described hereinabove. Rather, the scopeof the present invention includes both combinations and subcombinationsof the various features described hereinabove, as well as variations andmodifications thereof which would occur to persons skilled in the artupon reading the foregoing description and which are not disclosed inthe prior art.

1. Adaptive spectacles, comprising: a spectacle frame; first and secondelectrically-tunable lenses, mounted in the spectacle frame and havingrespective focal powers and optical centers that are determined bycontrol voltages applied thereto; and control circuitry, which isconfigured to apply the control voltages to the electrically-tunablelenses so as to shift the optical centers of the electrically-tunablelenses responsively to the focal powers of the electrically-tunablelenses.
 2. The spectacles according to claim 1, wherein the controlcircuitry is configured to shift the optical centers of the first andsecond electrically-tunable lenses downward when the focal powers of theelectrically-tunable lenses are increased.
 3. The spectacles accordingto claim 1, wherein the control circuitry is configured to reduce adistance between the optical centers of the first and secondelectrically-tunable lenses when the focal powers of theelectrically-tunable lenses are increased.
 4. The spectacles accordingto claim 1, wherein each of the first and second electrically-tunablelenses comprises: an electro-optical layer, having an effective localindex of refraction at any given location within an active area of theelectro-optical layer that is determined by a voltage waveform appliedacross the electro-optical layer at the location; and conductiveelectrodes disposed over opposing first and second side of theelectro-optical layer, wherein the control circuitry is coupled to applythe control voltages to the conductive electrodes.
 5. The spectaclesaccording to claim 4, wherein the electro-optical layer comprises aliquid crystal.
 6. The spectacles according to claim 4, wherein thecontrol circuitry is configured to apply the control voltages betweenthe conductive electrodes so as to generate a phase modulation profilein the electro-optical layer that causes rays of optical radiation thatare incident on the electrically-tunable lenses to converge or divergewith a given focal power, while varying an amplitude of the controlvoltages for the given focal power responsively to an angle of incidenceof the rays that impinge on the electrically-tunable lenses from adirection of interest.
 7. The spectacles according to claim 4, whereinthe control circuitry is configured to change the electrically-tunablelenses from a first focal power to a second focal power by concurrentlyapplying overshoot control voltages to each of a plurality of theconductive electrodes for different, respective transition periods,followed by application of the control voltages corresponding to thesecond focal power.
 8. The spectacles according to claim 4, wherein thecontrol circuitry is configured to apply the control voltages betweenthe conductive electrodes so as to generate a phase modulation profilein the electro-optical layer that causes rays of optical radiation thatare incident on the electrically-tunable lenses to converge or divergewith a given focal power, and to change the electrically-tunable lensesfrom the given focal power to zero focal power by concurrently applyinga predefined maximum high voltage to all of the conductive electrodes onthe first side of the electro-optical layer, followed by application ofa predefined low voltage thereto.
 9. The spectacles according to claim4, wherein the control voltages comprise waveforms selected so as tocause the electrically-tunable lenses to function as Fresnel lenses. 10.An optical method, comprising: providing spectacles including first andsecond electrically-tunable lenses mounted in a spectacle frame andhaving respective focal powers and optical centers that are determinedby control voltages applied thereto; receiving an input indicative of adistance from an eye of a person wearing the spectacles to an objectviewed by the person; and responsively to the distance, automaticallymodifying the control voltages so as both to tune the focal powers andto shift the optical centers of the electrically-tunable lenses.
 11. Themethod according to claim 10, wherein automatically modifying thecontrol voltages comprises shifting the optical centers of the first andsecond electrically-tunable lenses downward when the distance is lessthan a predefined threshold distance.
 12. The method according to claim10 wherein automatically modifying the control voltages comprisesreducing a distance between the optical centers of the first and secondelectrically-tunable lenses when the distance is less than a predefinedthreshold distance.
 13. The method according to claim 10, wherein eachof the first and second electrically-tunable lenses comprises: anelectro-optical layer, having an effective local index of refraction atany given location within an active area of the electro-optical layerthat is determined by a voltage waveform applied across theelectro-optical layer at the location; and conductive electrodesdisposed over opposing first and second side of the electro-opticallayer, wherein the control voltages are applied to the conductiveelectrodes.
 14. The method according to claim 13, wherein theelectro-optical layer comprises a liquid crystal.
 15. An optical method,comprising: providing spectacles including first and secondelectrically-tunable lenses mounted in a spectacle frame and havingrespective focal powers and optical centers that are determined bycontrol voltages applied thereto; and applying the control voltages soas to shift the optical centers of the electrically-tunable lensesresponsively to the focal powers of the electrically-tunable lenses. 16.The method according to claim 15, wherein applying the control voltagescomprises shifting the optical centers of the first and secondelectrically-tunable lenses downward when the focal powers of theelectrically-tunable lenses are increased.
 17. The method according toclaim 15, wherein applying the control voltages comprises reducing adistance between the optical centers of the first and secondelectrically-tunable lenses when the focal powers of theelectrically-tunable lenses are increased.
 18. The method according toclaim 15, wherein each of the first and second electrically-tunablelenses comprises: an electro-optical layer, having an effective localindex of refraction at any given location within an active area of theelectro-optical layer that is determined by a voltage waveform appliedacross the electro-optical layer at the location; and conductiveelectrodes disposed over opposing first and second side of theelectro-optical layer, wherein the control voltages are applied to theconductive electrodes.
 19. The method according to claim 18, wherein theelectro-optical layer comprises a liquid crystal.